Paving machines are used to apply, spread and compact a mat of material relatively evenly over a desired base. These machines are regularly used in the construction of roads, parking lots and other areas where a smooth durable surface is required for cars, trucks and other vehicles to travel. A paving machine generally includes a hopper for receiving paving material from a truck and a conveyor system for transferring the paving material rearwardly from the hopper for discharge onto a roadbed base. Screw augers may be used to spread the paving material transversely across the base in front of a screed assembly. The screed assembly smoothes and somewhat compacts the paving material and, ideally, leaves a mat of uniform depth and smoothness.
The screed assembly is drawn behind the paving machine by a pair of pivotally mounted tow arms. The screed assembly includes a main screed and one or more screed extensions disposed behind (or, in some embodiments, in front of) and adjacent to the main screed. The extension(s) are slidable transversely to the direction of travel of the paving machine and allow varying widths of paving material to be laid.
Road mat thickness is determined, in part, by the position of the tow arms and the angle of attack of the screed assembly relative to the base. To pave an even surface, the trailing edge of the main screed, and at least the inner end of the trailing edge of the screed extension should remain in the same plane. A change in the vertical height of the tow arms may cause the trailing edge of the main screed to be disposed at a different elevation than the trailing edge of the screed extension, at least temporarily. This difference in elevation can cause inconsistencies or discontinuities in the paved mat.
U.S. Pat. No. 6,352,386 (“Heims”) issued Mar. 5, 2002 describes a road finisher having a chassis and a floating laying beam that includes a basic beam and an extendable beam. The laying beam is attached to the chassis by tie bars. Each tie bar is pivotally articulated on its front end on the chassis. The point of articulation is adjustable in height with respect to the chassis.
Heims discloses height adjusting devices that keep the respective rear edges of the basic beam and of the extendable beam in the same plane. According to Heims, a pair of sensors is mounted on the tow arm. The sensors are offset from one another in the direction in which the mat is being laid. The sensor offset is the same as the offset distance between the basic beam and the extendable beam. In addition, the sensors are arranged at such a height above the reference plane, the underlying carriageway, that they measure the same vertical distance from the reference plane when the surfaces of the basic beam and of the extendable beam run parallel with respect to the reference plane. According to the disclosure, with any desired setting angle alpha, the difference in height between the sensors mounted on the tow arms corresponds to a height difference X between the extendable beam and the basic beam and is used to derive a height correction for the extendable beam. While such an arrangement may be beneficial, precise alignment and calibration is needed. Inaccurate readings may occur due to close proximity of the sensors to vibrating or rotating parts, inaccurate calibration during machine set up, and positional movement of the sensors or of the basic beam and extendable beams caused by normal wear and tear over time at paving work sites. A better design is needed.